Forming of a metallic layer onto a substrate bearing a thin conductive layer, usually copper, in an electrolyte environment, is implemented to form conductive lines during ULSI (ultra large scale integrated) circuit fabrication. Such a process is used to fill cavities, such as vias, trenches, or combined structures of both by electrochemical methods, with an overburden film covering the surface of the substrate. It is critical to obtain a uniform final deposit film because the subsequent process step, commonly a planarization step (such as CMP, chemical-mechanical planarization) to remove the excess conductive metal material, requires a high degree of uniformity in order to achieve the equal electrical performance from device to device at the end of production line.
Currently, metallization from electrolyte solutions is also employed in filling TSV (through silicon via) to provide vertical connections to the 3-D package of substrate stacks. In TSV application, via opening has a diameter of a few micrometers or larger, with via depth as deep as several hundreds of micrometers. The dimensions of TSV are orders of magnitude greater than those in a typical dual damascene process. It is a challenge in TSV technology to perform metallization of cavities with such high aspect ratio and depth close to the thickness approaching that of the substrate itself. The deposition rates of metallization systems designed for use in typical dual damascene process, usually a few thousand angstroms per minute, is too low to be efficiently applied in TSV fabrication.
To achieve the void-free and bottom-up gapfill in deep cavities, multiple organic additives are added in the electrolyte solutions to control the local deposition rate. During deposition, these organic components often break down into byproduct species that can alter the desired metallization process. If incorporated into deposited film as impurities, they may act as nuclei for void formation, causing device reliability failure. Therefore, during the deposition process high chemical exchange rate of feeding fresh chemicals and removing break-down byproducts in and near the cavities is needed.
In addition, with high aspect ratio, vortex is formed inside the cavities below where steady electrolyte flow passes on top of the cavity openings. Convection hardly happens between the vortex and the main flow, and the transport of fresh chemicals and break-down byproducts between bulk electrolyte solution and cavity bottom is mainly by diffusion. For deep cavity such as TSV, the length for diffusion path is longer, further limiting the chemical exchange within the cavity. Moreover, the slow diffusion process along the long path inside TSV hinders the high deposition rate required by economical manufacturing.
The maximum deposition rate by electrochemical methods in a mass-transfer limited case is related to the limiting current density, which is inversely proportional to diffusion double layer thickness for a given electrolyte concentration. The thinner the diffusion double layer, the higher the limiting current density, thus the higher the deposition rate possible. Various means to enhance fluid agitation to reduce the diffusion double layer thickness has been disclosed.
One method disclosed by U.S. Pat. No. 7,445,697 and WO/2005/042804 is by oscillating a series of paddles, termed as “shearplate”, near the stationary substrate surface of interest. It is recited that at 800 repetitions of these paddles the double layer thickness can be as thin as 10 micrometers. Although thinning the boundary layer thickness improves the deposition rate, the uniformity of deposited film is difficult to control since the substrate does not rotate.
Another fluid agitation method that has been widely disclosed is ultrasonic agitation, i.e. U.S. Pat. No. 6,398,937 and U.S. Pat. No. 5,965,043. This method is commonly practiced in various electrochemical metallization applications including printed circuit boards (PCB) and substrate packaging processes. Metallization of copper under ultrasonic agitation has drawn particular attention due to its importance in TSV applications (“The influence of ultrasonic agitation on copper electroplating of blind-vias for SOI three-dimensional integration” By Chen, Q. et. al. Microelectronic Engineering, Vol 87(3), Pages 527-531, 2010).
Although ultrasonic agitation further reduces the diffusion double layer thickness by forming acoustic streaming layer near reacting surface and by local cavitation bubble implosion, it does not provide uniform treatment to fluid near reacting surface. The nature of the acoustic wave propagation and its combination with reflected wave cause different energy dosage at different locations on reacting surface. The local deposition rate is not only a function of ultrasonic frequency but also directly related to the energy dosage at that point. This standing wave phenomenon leads to areas of various deposition rate across reacting surface. Above the energy threshold which cavitation will occur, bubble implosion takes place in a more or less random fashion, making overall process control very difficult.
Applying ultrasonic to electrochemical processes for the enhancement of mass transport has been well studied. Correlation between limiting current and operating parameters of an ultrasonic source has been established in a paper entitled “Transport Limited Currents Close to an Ultrasonic Horn Equivalent Flow Velocity Determination”, by B. G. Pollet et. al. in Journal of The Electrochemical Society, Vol. 154 (10), pp. E131-E138, 2007. The acoustic intensity (or energy dosage) received by reacting surface is sensitive to the gap between the ultrasonic source and reacting surface; hence, the limiting current density varies with that gap. This presents a greater challenge to forming uniform deposition using previously disclosed ultrasonic-assisted (UA) deposition methods (US 2008/0271995 and US 2007/0170066). In practice of substrate metallization process, the substrate rotation plane and the surface of ultrasonic source cannot be perfectly in parallel, largely due to mechanical tolerance in planar fixation and vertical alignment of rotation axis, as well as substrate warping itself. Thus within-substrate-uniformity of the deposited film is hard to control during such metallization processes.
UA metallization is an attractive method to be applied to processes such as filling TSV where rapid metallization and high chemical exchange rate are required. The diffusion double layer thickness in UA metallization can be reduced to a much smaller value than other methods, such as rotating substrate at high rpm or oscillating paddlers at substrate surface, therefore a higher deposition rate is warranted. The local agitation by acoustic stream and bubble implosion also create mass transport means other than diffusion inside deep vias and thus increase material exchange rate there.
With this method; however, a way of controlling deposition uniformity must be found for it to be applied to aforementioned processes.